Has to be the best way Peter but it will depend how deep the hole is.
I have a similar issue at times and have bored 5mm x 22mm using a 9mm sq HSS tool-bit ground away leaving a 3mm wide tip and about 4mm body, having first drilled out to 4mm. I'm sure you could do even better with a 3mm dia HSS tool-bit with the body ground away to (say) 2.5mm for clearance. If you drill out to 3mm first you are only going to be taking
0.005 or 0.010mm cuts anyway so there shouldn't be an issue of bending.
How you hold the tool-bit could be an issue but not to a man of your experience.
304 stainless for now, later 316L stainless and Inconels, maybe even something more exotic.
I don't follow.
I planned to drill a small through hole and then expand it with a boring bar, if I can get one that size - but I am open to other ideas.
It's an open ended hole, sort-of: a bit of 18 mm diameter scrap steel tube is mounted directly on the lathe spindle, faced off, and 6mm/8mm thick sheet blanks about 50 mm square are silver soldered to it for turning. They get made round and faced, then turned to shape, largest final diameter is 41mm.
I'm making small turbine wheels and guide vanes. The hole is cut, then the blades. Part.
To turn the back face I plan to use the hole to fit the seperated piece on a turned-in-place shaft. Accuracy is needed, and extreme concentrencity (sp?) of the hole is essential.
The holes will later be reamed out to final size, some will take shafts. The precise initial size of the hole doesn't matter - it has to be accurately perpendicular to the turned face, and accurately concentric, but that's all.
On my little Atlas, the boring bar holder accepts 1/2" round bars with sleeves for 3/16" 1/4" and 3/8". When I needed a 1/8"(.125") boring bar, I grabbed a piece of 1/4" dia. drill rod, turned down about an inch of it to .100", then ground a cutting end I could live with. Then hardened and honed the cutting end. You can also use a good quality bolt and do the same thing if you are fresh out of drill rod.
Suitably mounted, disposable milling cutters or slot drills make excellent small diameter boring bars. No regrinding needed - you already have several ready sharpened teeth. Just pick a size about 75% of final hole size.
For really tough jobs use a carbide end mill - they're stiffer than any conventional boring bar of similar size.
That's the place I was trying to think of. I bought a couple of them a few years ago -never had to use them, but thought they would be worth the money in case I ever needed them. They look great, and being carbide I'd expect them to cut well. Have never seen boring bars so small.